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  1. #1

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    Petri Lightmeter

    I was given an Agfa Isolette 1 by a friend some time ago who has no interest in photography so I was the lucky recipient. It had belonged to a deceased relative. There is no built in light meter as you will know. However in clearing out his loft he came across the above lightmeter which had been bought for the camera. It has a hotshoe attachment for the Agfa.

    There are no instructions with it. It's obviously a selenium meter and still works. The needle moves according to the amount of light it detects so in the shade it moves about a quarter of its range and in overcast evening daylight as now it moves to just over the half way point. In bright sunshine and pointed to the sky I imagine the needle will go nearly all the way.So it appears to cater for the full range of light

    There are two independent dials on it. The first dial has two windows for film speed in ASA and DIN. I presume you set this to whatever the film speed is. On the inner part of this rotating front there is a red pointer which is painted into the rotating front. As you change the film speed this red pointer obviously moves. Where the red pointer ends up in relation to the needle clearly depends on what film speed has been chosen.The inner dial consists of a series of shutter speeds(1/500th to 1 sec) and f stops(f2 to f22) which are fixed in relation to each other.

    What I cannot figure out is how you relate the needle position to the correct f stop and shutter speed for the prevailing light conditions.

    Presumably something needs to be lined up with the needle but what?

    FWIW the needle dial has a zero at its extreme left followed by alternating black and silver stripes and a thin black line which indicates the needle stop at the extreme right.

    Can anyone help here?

    Thanks

    pentaxuser

  2. #2
    Jim Jones's Avatar
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    A picture would be worth a few hundred words here. Or give us the make and model of the meter, and we can perhaps look it up.

  3. #3

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    Quote Originally Posted by Jim Jones
    A picture would be worth a few hundred words here. Or give us the make and model of the meter, and we can perhaps look it up.
    Thanks Jim. I posted this knowing I'd be away for a few days and hoped that I'd have several replies at least but yours is the only one.

    Unfortunately it only says The Petri camera company inc Japan. If the meter was bought as an accessory to the camera, I can only assume it is of a similar age. So probably 45-50 yrs old. It fits into the hotshoe on top of the Agfa in the same way separate rangefinders did for cameras that otherwise had to rely on the user's judgement about distance.

    There's no model number at all. Both dials can be turned continuously in either direction i.e. there are no end stops on either dial. The meter is cylindrical in shape, about 2 inches long and the front glass which gathers the light is about an inch across and has concentric raised rings. The glass part is set into the first dial about a quarter of an inch so it looks like a lens hood.

    Could it be that it is meant to show the correct exposure at a set film speed(say ASA50) and fstop(say f5.6) by setting this stop opposite the needle, where ever that falls and ASA 50 on the front dial which has a fixed red arrow. I have just tried it like this in average room artificial light and this gives me about 1/4 sec. If the actual film speed is greater, say 400 this moves the arrow to about 1/30th sec and if I then alter the f stop to say f2 this moves the shutter speed to about 1/150th. Moving the fstop dial to align a bigger f stop to the needle also moves the shutter speed. As previously described the inner dial has both shutter speeds and f stops on it in a fixed relationship. So everything moves the right way based on the relationship between film speed, f stop and shutter speed.

    So to summarise the outer film speed dial has the fixed red arrow and inner dial has the shutter speeds and f stops.

    I cannot work out any other way for it to work except on the principle that the needle is accurate for a certain predetermined film speed and f stop and all other film speeds and f stops can be determined by simply moving the dials. It all could be done mentally but the two dials for film speed and shutter speed and f stops act a bit like the old cylindrical slide rule which gave answers without the user performing mental calculations.

    I'll compare this to my pentax camera meter readings tomorrow in daylight to see if this the theory works.

    However it would be nice to know what the predetermined f stop and film speed are.

    The reason I say f 5.6 and ASA 50 is that I have a light meter of a similar or slightly earlier vintage which works like looking through a small telescope and reading the dimmest shutter speed figure which is based on f 5.6 and a film speed of ASA50 which was probably the average film speed of the early to mid 1950s.

    Does this logic make sense to you?

    Thanks

    pentaxuser

  4. #4

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    I tried the Petri meter today in overcast daylight according to what I suggested might be the rule i.e. the needle being based on an f stop of 5.6 and an ASA of 50. However based on the meter reading of my Pentax, this must be wrong. I now think that the needle reading is based on something close to the sunny f16 rule. So setting the dial at f16 opposite the needle and using the same ASA as I had set in the Pentax which was ASA400 gave a very similar but not identical exposure reading.

    I am sure that unlike the meter you hold to your eye like a telescope which I know to be based on f5.6 and a ASA of 50, the Petri meter is not based on this but I can't be certain it is based on the sunny f16. Only that it is close to this.

    It sounds as if this is a conclusion on my part but it isn't. I am still interested in any information on the Petri meter.

    Thanks

    pentaxuser



 

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